It's the ammonia level. You need to get that down as soon as possible. If the water change did not get it back down to 0, go out and purchase a bottle of Fritz-Zyme Turbo Start. This stuff will release a lot of beneficial bacteria into your tank, thus converting the ammonia into nitrates and putting the ammonia back to 0. The ammonia is causing the oxygen exchange to drop, thus the fish are probably having trouble breathing. If they can't breathe, they probably aren't very interested in eating either.
You do need to look into this ammonia problem though. You are lucky the levels are not higher and that your fish are still alive. How often do you feed them and how much? How often do you do water changes?
